Vapor deposition process

ABSTRACT

A vacuum vapor deposition process is described for depositing a coating of a cobalt-base alloy on a substrate. The quality of the deposited coating is improved by maintaining the substrate above a minimum temperature of 1,600*F, in the case of alloys with substantially soluble constituents, and of 1,700*F, in the case of alloys having at least partially insoluble constituents.

United States Patent [191 dA. Hunt [451 Mar. 19, 1974 VAPOR DEPOSITION PROCESS [75] Inventor: Charles dA. Hunt, Moraga, Calif.

[73] Assignee: Airco, Inc., New York, NY.

[22] Filed: Dec. 27, 1971 [21] Appl. No.: 212,250

Related U.S. Application Data [63] Continuation-impart of Ser. No. 790.500, Dec. 13,

1968, Pat. No. 3,652,325.

[52] U.S. Cl. 117/107, 117/71 M [51] Int. Cl. C236 13/02 [58] Field of Search 117/50, 107.2, 107, 107.1,

[56] References Cited UNITED STATESPATENTS 3,560,252 2/1971 Kennedy 117/107 9/1960 Tellkamp 117/107 7/1957 Kohring 117/107 Primary Examiner-Alfred L. Leavitt Assistant Examiner.l. Massie I Attorney, Agent, or Firm-Fitch, Even, Tabin &

Luedeka 5 7 ABSTRACT A vacuum vapor deposition process is described for depositing a coating of a cobalt-base alloy on a substrate. The quality of the deposited coating is improved by maintaining the substrate above a minimum temperature of 1,600F, in the case of alloys with substantially soluble constituents, and of 1,700F, in the case of alloys having at least partially insoluble constituents.

3 Claims, 3 Drawing Figures PATENTEU MAR l 9 I974 F/sQ/ VAPOR DEPOSITION PROCESS This application is a continuation-in-part of copending application Ser. No. 790,500 filed Dec. 13, l968 now US. Pat. No. 3,652,325.

This invention relates to vacuum vapor deposition and, more particularly, to vacuum vapor deposition of cobalt-base alloys in a manner which provides a coating of improved quality.

The technique of evaporating and condensing various materials on a substrate in a vacuum is known in the art as vacuum vapor deposition. Vacuum vapor deposition may be utilized to produce coatings on substrates for various purposes. Iron-base alloys and cobalt-base alloys may be utilized in coatings produced by vacuum vapor deposition for a number of reasons, one significant reason being to provide resistance against corrosion. For example, turbine blades subject to high temperature operation may be comprised of nickel or a material having similar high strength properties at elevated temperatures. Nickel and similar materials, however, are sometimes subject to excessive corrosion, and a coating of a corrosion resistant iron-base or cobalt-base alloy may be provided for protection.

Coatings that are designed to protect a substrate against corrosion should be relatively free of defects, such as microscopic cracks and similar discontinuities, or corrosion will penetrate the coating and attack the substrate, possibly leading to failure. In producing a coating by means of vacuum vapor deposition, defects in the coating may be caused by three factors:

I. Flaws and other discontinuities previously existing in the surface of the substrate;

2. Particles, such as droplets of splattered liquid metal or flakes of condensate, landing on the substrate during coating; and

3. Long continuous grain boundaries in the coating extending in a straight line from the outer surface of the coating to the substrate surface.

Flaws in the surface of the substrate may consist of any part of the substrate surface which is not substantially parallel with the remainder of the substrate surface. A step, indentation or scratch in the substrate surface may constitute such a surface flaw that will be continued as a discontinuity all the way to the surface of the deposited coating. In some cases, a flaw on the substrate surface may even be magnified in the formation of the discontinuity in the deposited coating. Microscopic cracks in the coating often develop between grains adjacent such a discontinuity. This often leads to very rapid attack of the substrate if subjected to a corrosive environment.

In addition to substrate surface flaws, particles landing on the part during the coating process may also produce discontinuities in the coating. Such particles may consist of splattered droplets from the molten liquid which is being evaporated, or may comprise flakes of condensate which fall off of cooled parts within the vacuum vapor deposition furnace and land on the part. Such a particle may not create a discontinuity that extends all the way from the surface of the coating to the substrate. Nevertheless. the discontinuity which is produced. may cause development of microscopic cracks in the coating as in the case of substrate surface flaws. This decrease in the integrity of the coating makes the coating and the substrate it covers more susceptible to attack by corrosion.

Grain boundaries of evaporated and condensed material generally appear to be substantially weaker mechanically and more susceptible to corrosion than are grain boundaries found in metal which is frozen directly from a molten state. Frequently, in vacuum vapor deposition coatings, the grains appear to grow almost independently and the grain boundaries have little or negligible strength. If, in addition to having generally weaker grain boundaries, the coating is of a very columnar grain structure in which the grain boundaries extend continuously in a straight line from the outer surface of the coating to the substrate surface, the substrate is susceptible to corrosive attack by preferential corrosion along the grain boundaries.

It is known that the physical characteristics of a vacuum vapor deposited coating may be influenced by the temperature of the surface of the substrate upon which the coating is deposited. A process in which the substrate surface is maintained at an elevated temperature during deposition is described in US. Pat. No. 3,270,381 wherein foil of improved ductility is produced. The minimum temperature above which the substrate surface is maintained in the aforesaid patent is of at least about 25 percent of the absolute melting point of the coating material. As pointed out in that patent, the reason for the transformation of the deposited coating from a brittle condition to a ductile condition at substrate temperatures above 25 percent of the absolute melting point of the coating material is not fully understood. The deposited coating under such conditions has a columnar grain structure when deposited on a substrate maintained at a temperature either slightly above or slightly below 25 percent of the absolute melting point of the coating material and it is difficult to detect any difference in the deposited coatings under magnification.

In the previously mentioned copending application, a process was described for producing coatings of ironbase alloys. It may be preferable under some circumstances to use a coating of a cobalt-base alloy rather than an iron-base alloy.

It is the principal object of this invention to provide an improved process for coating a substrate with a cobalt-base alloy.

Another object of the invention is to provide a process of vacuum vapor deposition of cobalt-base alloys wherein substrate surface flaws and particle inclusions in the coating do not result in a substantial reduction of the integrity of the coating.

It is another object of the invention to provide a coated substrate product having a vacuum vapor deposited coating of improved quality.

Other objects of the invention will become apparent to those skilled in the art from the following description taken in connection with the accompanying illustrations wherein:

FIG. 1 is a cross sectional photomicrograph, enlarged 200 times, of a nickel alloy turbine blade coated, at a substrate temperature of l,200F, with a cobalt-base alloy, vacuum heat treated, peened and air annealed;

FIG. 2 is a cross sectional photomicrograph, enlarged 200 times, of a nickel alloy turbine blade coated, at a substrate temperature of l,500F, with a cobalt-base alloy, vacuum heat treated, peened and air annealed; and

FIG. 3 is a cross sectional photomicrograph, enlarged 200 times, of a nickel alloy turbine blade coated, at a substrate temperature of 1,750F, with a cobalt-base alloy, vacuum heat treated, peened and air annealed.

Very generally, the method of the invention comprises vaporizing the cobalt-base alloy in a vacuum and condensing at least a portion of the vaporized alloy on the substrate. The surface of the substrate upon which the alloy is condensed is maintained at a temperature which is above a minimum temperature during condensation of the alloy. In the case of alloys having substantially soluble constituents at the deposition temperature, the minimum temperature is about 1,600F. In the case of alloys having at least partially insoluble constituents at the deposition temperature, the minimum temperature is about 1,700F.

The method of the invention may be practiced in a vacuum-tight enclosure evacuated to a low pressure, for example less than one Torr. The substrate upon which a coating is to be deposited is disposed in the enclosure. The substrate may be a flexible web or sheet of material passed from a supply roller to a take-up roller, or may be a discrete object suitably supported and, if desired, rotated within the vapor flow. Vacuum valves may be provided in the walls of the enclosure through which the substrate may be passed into and out of the enclosure.

The cobalt-base alloy which is to be deposited on the substrate may be contained within a crucible. The crucible may be comprised of refractory material, however, it is preferred that the crucible be water cooled copper or stainless steel and that heating of the material therein be accomplished by means of electron beams. in this manner, high purity and close control of coating composition may be achieved. Apparatus for vapor deposition which is constructed generally in accordance with the foregoing description is shown and described in US. Pat. No. 3,l83,563, assigned to the assignee of the present invention. Such apparatus may be utilized for carrying out the method of the present invention in order to produce the product of the invention.

In practicing the method of the invention, the substrate is disposed in a vacuum chamber and the cobaltbase alloy to be deposited thereon is vaporized. At least a portion of the evaporated cobalt base alloy is allowed to condense on the substrate. Prior to and during con-- densation, the surface of the substrate is maintained above a minimum temperature. The temperature is se lected to be sufficiently high as to impart enough mobility to to the atoms of the deposit to overcome the barriers of potential energy present in the crystalline deposit structure so that the condensing atoms have a tendency to seek a smooth surface. This conforms with the laws of physics which indicate that atoms, when mobilized, tend to seek the conditions of least energy.

Heating of the substrate may be accomplished by a heater of the resistance or inductance type or, if desired, by an electron beam gun which directs a beam of electrons at the uncoated side of the substrate or at a radiant heater element placed nearby. The arrangement of the heaters should be such as to avoid heating the substrate to a temperature at or exceeding the reemission temperature of the evaporant, since under the latter condition no condensation will occur.

If heating is provided to impart sufficient mobility to the atoms of the deposit, not only will the deposit tend to cover up surface flaws in the substrate, but particles landing on the coating during the deposition process are also often masked. Moreover, the tendency for microscopic cracks to develop as a result of discontinuities in the coating is reduced. Furthermore, improved strength in grain boundaries appears to occur throughout the deposit when produced in accordance with the invention, thereby rendering the deposit less susceptible to selective corrosion along grain boundaries, and less susceptible to flaking, peeling or chipping.

For some cobalt-base alloys, condensation at substrate temperatures that are elevated in accordance with the invention will not only provide superior grain boundary strength and integrity, and a superior ability to cover up surface defects and particles, but will also produce a random distribution of the grains in the coating. For many materials which normally tend to have a columnar distribution of grains when vapor deposited, a transition temperature exists above which the distribution of the grains is generally random. Such transition temperatures generally exceed about 50% of the absolute melting temperature of the deposit for cobaltbase alloys having isotropic crystalline structures at their condensation temperatures and which, after condensation, form substantially homogeneous solid solutions. Such cobalt-base alloys include cobalt'nickel and cobalt-chromium alloys. This is typically true also for pure cobalt. Deposits in which the grains are randomly distributed provide a significant advantage from the standpoint of resisting corrosion. This is because there are no grain boundaries which extend in a generally straight line from the surface of the substrate to the surface of the coating and along which preferential grain boundary corrosion may occur.

It has been found that for improved but not necessarily random-grain coating of those cobalt-base alloys comprised of constituents which are substantially soluble atthe deposition temperature, the minimum temperature at which the surface of the substrate should be maintained is about 1,600F for satisfactory results. On the other hand, where insoluble phases are formed at the deposition temperature, such as in the case of yttrium and the rare earth metals, it is typically necessary to exceed about l,700F for satisfactory results. In either circumstance, vacuum vapor deposition carried out in accordance with the invention provides a coating in which microscopic cracks resulting from discontinuities propagated from surface defects and particle inclusions are minimized. Moreover, the integrity of the grain boundaries in coatings produced in accordance with the invention appears superior to coatings obtainable with heretofore known methods. This is true even though grain distribution is not random, although the latter situation may provide additional corrosion resistance due to the avoidance of long grain boundaries extending from the surface of the deposit to the substrate.

posits may be attained in accordance with the invention include cobalt-nickel, cobalt-copper and cobaltchromium alloys. Cobalt-base alloys having insoluble phases present at the deposition temperature and for which superior quality deposits may be attained in accordance with the invention include cobalt-chromiumaluminum, cobalt-aluminum-yttrium, cobaltgadolinium, and cobalt-titanium alloys.

Referring now to the FIGURES, a case of mutually insoluble phases being present at the deposition temperatures is illustrated. In this case, the alloy comprises 25 percent chromium, 8 percent aluminum, 0.4 percent yttrium, balance essentially cobalt. In FIG. 1, the deposition temperature was 1,200F. It may be seen that a deep layer of poorly defined grain structure is present along with inclusions of fine spherical particles. This is clearly an unsatisfactory deposit because of its lack of homogeneity. In FIG. 2, the substrate temperature was about 1,500F. Although the layer of poor grain structure is not as deep as in FIG. 1, it is still present and therefore unsatisfactory. In FIG. 3, the substrate temperature was I,750F. A clear uniform grain structure from interface to surface may be seen. As a result, superior corrosion resistance occurs, indicating that the integrity of the grain boundaries is improved. Masking properties may be enhanced by more elevated substrate temperatures, such as about 2,000F.

Further evidence of the integrity of the coating of FIG. 3 as opposed to the coatings of the other two FIG- URES occurred during the peening process. Peening and subsequent annealing has been found to improve the surface quality of the turbine blades. During peening of blades coated as in FIGS. 1 and 2, however, 30

percent to 70 percent of the coating chipped off in places where the coating angle was steep or where the coating was subject to higher stresses. On the other hand, coatings made as in FIG. 3 are capable of withstanding the peening process without significant chip- 7 ping even where the deposit is placed on at a high deposition angle or is subject to high stresses.

It may therefore be seen that the invention provides an improved method for vacuum evaporating and depositing cobalt-base alloys on a substrate. The development of microscopic cracks is substantially avoided, even in the presence of substrate surface imperfections and particle inclusions. Moreover, the integrity of the grain boundaries is superior and thereby provides superior corrosion protection for the substrate. In some cases, the grain distribution of the coating is random, enhancing the ability of the coating to withstand corrosion. The invention is applicable in depositing several successive layers of cobalt-base alloys of different compositions, rather than the single layer.

Various modifications of the invention will become apparent to those skilled in the art from the foregoing description and accompanying illustrations. Such modifications are intended to fall within the scope of the appended claims.

What is claimed is:

l. A process for coating a substrate with a cobaltbase alloy containing at least one of the elements nickel, copper, chromium, aluminum, yttrium, gadolinium and titanium, comprising: vaporizing the cobaltbase alloy in a vacuum, condensing at least a portion of the vaporized alloy on the substrate, and maintaining the surface of the substrate upon which the alloy is condensed above a minimum temperature during condensation of the alloy, said minimum temperature being about 1,600F in the case of alloys having only at least one of the substantially soluble constituents nickel, copper and chromium at the deposition temperature, and being about 1,700F in the case of alloys having at least one of the partially insoluble constituents aluminum, yttrium, gadolinium and titanium, at the deposition temperature.

2. A method according to claim 1 wherein the substrate surface is maintained at a temperature below that at which epitaxial growth will occur.

3. A method according to claim I wherein the coating formed is sufficiently thick to comprise, generally, at least two layers of grains.

Disclaimer 3,798,055.-0harles dA. Hunt, Moraga, Calif. VAPOR DEPOSITION PROCESS. Patent dated Mar. 19, 1974. Disclaimer filed Mar. 26, 197 4, by the assignee, Airco, Inc.

Hereby disclaims the portion of the term of the patent subsequent to Mar.

[OfiZcz'aZ Gazette Apm'l 23, 1.974.] 

2. A method according to claim 1 wherein the substrate surface is maintained at a temperature below that at which epitaxial growth will occur.
 3. A method according to claim 1 wherein the coating formed is sufficiently thick to comprise, generally, at least two layers of grains. 